Urea and urea-containing fertilizers presently account for at least about 30 percent of the fertilizer nitrogen applied in the United States [J. Darwin Bridges, Fertilizer Trends 1982, TVA (1983)], and urea accounts for as much as 60 percent of the fertilizer nitrogen applied worldwide (unpublished TVA data). The economics-based prediction for these percentages shows an increase because urea has a high nitrogen content, low transportation cost, and low production cost relative to alternative nitrogen sources, such as ammonium nitrite and ammonium sulfate. Inasmuch as the relative importance of urea as a primary nitrogen fertilizer is expected to increase to even greater proportions than it now enjoys and substantial amounts of such urea and/or urea-containing fertilizers are applied in situations such as reduced tillage, pastures, and nonmechanized agriculture where it is impractical to mechanically incorporate urea to prevent ammonia volatilization, it will be readily appreciated that the development and practice of using suitable urease inhibitors is an endeavor of considerable importance for both domestic and international agricultural considerations.
1. Field of the Invention
The concept underlying the gist of the instant invention is based on the unexpected discovery that thiophosphoryl triamide is more stable in slightly acidic, neutral, and slightly basic fluid fertilizers than it has been found to be in solid mixtures with urea.
2. Description of Prior Art
A number of chemical compounds have been patented or demonstrated effective as urease inhibitors. Perhaps the best-known urease inhibitor is phenyl phosphorodiamidate (PPDA), (C.sub.6 H.sub.5 O)PO(NH.sub.2).sub.2. Several researchers in the prior art have demonstrated that PPDA is an extremely potent inhibitor of urease activity [P. Held, S. Lange, E. Tradler, M. Klepel, D. Drohne, H. J. Hartbrich, G. Rothe, H. Scheler, S. Grundmeier, and A. Trautman, East German Pat. No. 122,177 (Cl. CO5G3/08, Sept. 20, 1976), Chem. Abstracts 87:67315W, D. A. Martens and J. M. Bremner, Soil Sci. Soc. Am. J. 48, 302-305 (1984)]. Because of its effectiveness PPDA has become the standard by which all other urease inhibitors normally are judged. Another phosphoramide, phosphoryl triamide (PTA), (NH.sub.2).sub.3 PO and a series of N-(diaminophosphinyl)arylcarboxamides were patened as urease inhibitors by Bayless and Millner [U.S. Pat. Nos. 4,242,325 (1980) and 4,182,881 (1980)]. Other researchers have shown that diamidophosphoric acid (DAPA), (NH.sub.2).sub.2 PO(OH), and monoamido phosphoric acid, MAPA, NH.sub.2 PO(OH).sub.2, are urease inhibitors [A. Barth, W. Rollka, and H. J. Michel; Wissenschaftliche Beitraege-Martin Luther Universitaet Halle Wittenberg, No. 2, pp 5-10 (1980); N. E. Dixon, C. Gazzola, J. J. Waters, R. L. Blakeley, and B. Zerner, J. Am. Chem. Soc. 97, 4131 (1975)]. More recently, a new compound, thiophosphoryl triamide (TPTA), (NH.sub.2).sub.3 PS has been demonstrated to be an urease inhibitor (R. J. Radel, U.S. Statutory Invention Registration No. H25, Feb. 4, 1986). Thiophosphoryl triamide has been shown by Radel supra to be comparable and in some cases more effective than PPDA in its urease inhibition effect.
In this area of the prior art relating to certain compounds and materials for purposes of urease inhibitor use, it has been shown to be highly desirable that the inhibitor material be intimately mixed with the fertilizer material with which it is to used. This procedure ensures the positional proximity to the fertilizer to inhibit urease activity. Therefore, in order for a urease inhibitor to be of practical use, it must be compatible with the fertilizer materials with which it is to be used. For example, if the inhibitor decomposes in the process of incorporation into fertilizer materials or decomposes in the fertilizer materials during storage prior to application to soil, then it is ineffective unless its degradation products are also effective inhibitors.
Solid urea for fertilizer use is normally produced by either the processes of prilling or of granulation. In order to use urease inhibitors in solid urea produced by such processes it has been found not only desirable, but indeed necessary to incorporate the inhibitor into the urea at the point of manufacture during either the prilling or the granulation process, it being understood that homogeneity of the end product is of the utmost importance. This consideration, of course, requires that the inhibitor be exposed to hot urea melts (140.degree. C.) for at least short periods of time (generally less than about five minutes). After cooling, the resulting solid urea urease inhibitor enriched granules are stored in huge piles in bulk storage sheds for periods of time which frequently range as long as from six months to one year.
The prior art teaches that urea-containing fluids can also be used as fertilizer. Such fluids usually comprise water, urea, clay (as a suspending agent), and other fertilizer materials such as ammonium nitrate. The most commonly used high nitrogen fluids containing urea are urea-ammonium nitrate (UAN) solution and suspension of grades 28-0-0 and 31-0-0. At the present time the Tennessee Valley Authority is developing two new high-analysis fluids containing urea, which fluids are in the form of suspensions. These fluids are an UAN suspension of grade 36-0-0 and an urea-ammonium sulfate suspension (UAS) of grade 29-0-0-5S. Fluid fertilizers have several advantages over solid fertilizers. Fluids allow more uniform application of fertilizer materials than solid forms, they are easy to transport and handle, and because they can be applied uniformly they are excellent carriers for micronutrients and pesticides.
In the practice of fluid fertilizer use, nitrogen base fluids such as UAN (28-0-0), UAN (31-0-0), UAN (36-0-0), and UAS (29-0-0-5S), and phosphorous base fluids such as 10-34-0, 9-32-0, 13-38-0, and 10-30-0 are usually produced at the fertilizer manufacturing site and are subsequently shipped to fertilizer dealers who in turn apply the fluids directly or custom mix them to suit the needs of individual farmers. Custom mixtures are usually applied within one week after preparation. Accordingly, it will be appreciated that this practice of fluid fertilizer use thus provides for but two options for addition of urease inhibitors to fluid fertilizers, i.e., addition at the manufacturing site, or addition by the fertilizer dealer just prior to the application thereof.
Studies of the feasibility of cogranulating PPDA with urea [J. Gautney, Y. K. Kim, and P. M. Gagen, I&EC Prod. R&D 23, 483-489, (1984)] showed that this inhibitor could be cogranulated with urea with only small losses of inhibitor during the granulation process. Later studies [J. Gautney, A. R. Barnard, D. B. Penney, and Y. K. Kim, "Solid-State Decomposition Kinetics of Phenyl Phosphorodiamidate," Soil Science Society of America Journal 50, 792-797, (1986)] showed that PPDA decomposes in the solid state and that the rate of PPDA decomposition is accelerated in urea mixtures. For example, at 25.degree. C. the half-life of PPDA is 254 years, whereas in mixtures with urea the half-life is only 0.9 years.
The results of decomposition kinetic studies with PPDA in fluid fertilizers indicate that this inhibitor also decomposes quite rapidly in fluid fertilizers [J. Gautney, Y. K. Kim, and A. R. Barnard, "Solubilities and Stabilities of the Nitrogen Loss Inhibitors Dicyandiamide, Thiourea, and Phenyl Phosphorodiamidate in Fluid Fertilizers," I&EC Prod. R&D 24, 155-161, (1985)]. Phenyl phosphorodiamidate decomposition rates were much faster in fluids than in solid mixtures with urea. In UAN (31-0-0) at 25.degree. C. the first-order reaction half-life ranged from 1.2 days at pH 8.14 to 6.1 days at pH 6.46. Decomposition rate constants for PPDA in UAN (36-0-0) and UAS (29-0-0-5S) saturated with PPDA at 25.degree. C. were 10.2 and 20.5 percent decomposition per day, respectively.
As a result of its instability, the use of PPDA in solid urea is generally considered impractical and its use in fluid fertilizers is severely limited inasmuch as delays of as little as one to two days (commonly encountered because of inclement weather) between addition of PPDA to the fluid fertilizer and application to the soil can result in significant inhibitor losses.
Fertilizer compatibility studies with PTA and TPTA (TVA Bulletin Y-191, "New Developments in Fertilizer Technology," October 1985) showed that PTA is much less stable in urea melt than PPDA and as a result probably cannot be cogranulated with urea without significant losses of inhibitor. Thiophosphoryl triamide, on the other hand, was found to be substantially more stable in urea melt than PPDA and a result can be cogranulated with urea with minimal inhibitor loss.
Compatibility studies with PTA and TPTA in solid mixtures with urea (J. Gautney, Y. K. Kim, R. J. Miles, and L. M. Mossburg, unpublished TVA data, 1985) gave reaction half-lives of 4.3 and 38.5 hours, respectively, at 25.degree. C. and a water partial pressure of 12.73 mm of Hg. As a result of the instability of PTA and TPTA, the use of these inhibitors in solid mixtures with urea, like that of PPDA, has to be considered impractical.
More recently, it has been quite unexpectedly discovered that TPTA is more stable in slightly acidic, neutral, and slightly basic fluid fertilizers containing urea than it is in solid mixtures with urea. This discovery forms the basis for the principal objects, findings, and teachings of the present invention.